Purification of process gas streams by hydrogenation

ABSTRACT

Process gases, for example, those gases derived from the pyrolysis or cracking of butane, kerosene, naphtha, refinery gases and other mineral oil and gas oil fractions, are excellent sources of olefins for the plastics industry. The acetylenes must of course be removed from these process gases before they are usable as raw materials for plastics. Hydrogenation is the process of choice, and a selective catalyst must be used so that acetylenes will be hydrogenated to the exclusion of the olefin. Partially sulfided catalysts afford a high degree of selectivity in hydrogenating acetylenes to the exclusion of olefins, but there is no selectivity between acetylenes and dienes, dienes being desired products. Accordingly the C4 stream is generally removed prior to the selective hydrogenation reaction. Butylene, butadiene, propylene, and ethylene are removed herein from a process gas stream also containing acetylene and methylacetylene and propadiene by hydrogenating the acetylenes with minimal hydrogenation of butylene, butadiene, propylene, and ethylene. The process gas stream is passed through a bed of a catalyst so that the acetylenes are incompletely hydrogenated, the butadiene thus remaining unsaturated. Subsequently the gas stream is separated into C2, C3, and C4 fractions, and then the C2, C3, and C4 fractions are conducted to separate hydrogenation zones.

United States Patent Livingston [54] PURIFICATION OF PROCESS GAS [73]Assignee: Catalysts and Chemicals Inc., Louisville,

[22] Filed: July 28, 1970 [21] Appl. No.: 58,887

' [451 July 25, 1972 ABSTRACT Process gases, for example, those gasesderived from the pyrolysis or cracking of butane, kerosene, naphtha,refinery gases and other mineral oil and gas oil fractions, areexcellent sources of olefins for the plastics industry. The acetylenesmust of course be removed from these process gases before they areusable as raw materials for plastics. Hydrogenation is the process ofchoice, and a selective catalyst must be used so that acetylenes will behydrogenated to the exclusion of the olefin. Partially sulfidedcatalysts afford a high degree of selectivity in hydrogenatingacetylenes to the exclusion of olefins, but there is no selectivitybetween acetylenes and dienes, dienes being desired products.Accordingly the C stream is generally removed prior to the selectivehydrogenation reaction. Butylene, butadiene, propylene, and ethylene areremoved herein from a process gas stream also containing acetylene andmethylacetylene and propadiene by hydrogenating the acetylenes withminimal hydrogenation of butylene, butadiene, propylene, and ethylene.The process gas stream is passed through a bed of a catalyst so that theacetylenes are incompletely hydrogenated, the butadiene thus remainingunsaturated. Subsequently the gas stream is separated into C C and Cfractions, and then the C C and C fractions are conducted to separatehydrogenation zones.

5 Claims, 1 Drawing Figure CARBON MONOXIDE [52] 11.8. CI. ..260/677 H,260/6815 [51] Int. Cl ..C07c 5/02 [58] Field of Search ..260/677 H [56]References Cited UNITED STATES PATENTS 3,003,008 10/1961 Fleming et al...260/677 H 3,471,583 10/1969 Fleming ....260/677 H 3,420,618 l/1969Fleming ..260/677 H 3,373,219 3/1968 Kronig ..260/68l.5

Primary Examiner-Delbert E. Gantz Assistant Examiner-Veronica OKeefeAtlorney-Norman L. Wilson, Jr.

HYDROGEN METHANE THIRD- STAGE L COMRESSION HYDROGEN SULFIDE FRONT- ENDn: CATALYTIC CONVERTER g CHILLER v [I] 2 Lu D FROM SECOND-STAGECOMPRESSION 1: LL] r 2 r3 r5 FOURTH STAGE 5 COMPRESSlON C plus TAIL-ENDCATALYTIC CONVERTER ETHYLENE (I L11 t l D. 7 (ll 3 i LU J E TAIL-END {I5 CATALYTIC CONVERTER N E ETHANE I 4 PROPYLENE F5 cc UJ II 1- g 5 5 Pk11 Lu 0 z 1! Lu 0. l .5 i E 2 D.

i PROPANE BUTADIENE, BUTYLENE, BUTANE PURIFICATION OF PROCESS GASSTREAMS BY HYDROGENATION BACKGROUND OF THE INVENTION This inventionrelates to the purification by selective hydrogenation of hydrocarbonstreams containing highly unsaturated compounds.

In one of the common methods for manufacturing olefins a hydrocarbonmixture such as refinery gas, naphtha, kerosene, or gas oil is passedthrough a reactor heated sufficiently to decompose the mixture with theformation of hydrogen and one or more unsaturated compounds. Pyrolysisprocesses of this type have been carried out at temperatures rangingfrom about 500 C. to about l,200 C. with the yield of olefinic productsper pass-through the reactor being highest at temperatures above 600 C.The olefin-containing mixtures obtained in such processes, generallyknown as process gases, usually contain a variety of other hydrocarbonsalong with the olefins. As an example, cracked butanes, cracked gas oilor refinery gas, all of which are well known sources of olefins, usuallycontain paraffinic hydrocarbons ranging from methane to hexane, olefinssuch as ethylene, propylene, butylene, amylene and hexenes, diolefinssuch as propadiene, l,3 butadiene, 1,2 butadiene, and small quantitiesof acetylenic hydrocarbons such as acetylene, methylacetylene, and thelike. The yield of acetylenic by-products becomes appreciable whenoperating at pyrolysis temperatures above 600 C., i.e., within thetemperature range at which the olefins are usually produced in maximumyield.

Normally acetylenes can be readily hydrogenated, acetylene to formethylene, or ethane, methylacetylene to form propylene or propane, etc.However, when olefins are present, the hydrogenation process is muchmore difficult to carry out. Selective hydrogenation poses a difficultproblem when small amounts of acetylenic compounds are present in anolefin gas mixture. Thus, gas mixtures consisting essentially ofethylene for commercial uses such as high polymeric plastics shouldcontain no more than about parts per million (ppm) acetylenic impuritiessuch as acetylene and methylacetylene. For the production ofpolyethylene, an ethylene stream is frequently demanded containing lessthan 5 ppm, and in some instances less than 1 ppm acetylene. Such beingthe case, virtually all of the acetylenes must be hydrogenated,preferably to olefins, without hydrogenation of olefins occurring also.However, as pointed out in US Pat. No. 2,5] 1,453, if it is desired toremove all but a trace of the acetylene originally present, acomparatively active catalyst must be used, with the result that theolefin content of the gas is appreciably reduced. If, in order to avoidthis loss of olefin, a less active catalyst is employed, then a highdegree of acetylene removal cannot be achieved.

In US. Pat. No. 2,511,453 it was shown that a catalyst is availablehaving a high degree of selectivity for the hydrogenation of acetylenesin the presence of lower olefins. This selectivity is accomplishedthrough the use of a partially sulfided, reduced nickel oxide catalyst,partially sulfided" designating catalysts containing 2 to 50 atomequivalents sulfur per 100 atom equivalents nickel.

Most process gas streams contain a C component. Among the hydrocarbonsin a C stream are butylene and butadiene, both of which are desiredmaterials. One of the disadvantages of the sulfided nickel catalysts isthat whereas they selectively hydrogenate acetylenes to the exclusion ofolefins, there is no selectivity between acetylenes and dienes.Butadiene is therefore hydrogenated along with the acetylenic compoundsand propadiene.

In order to recover ethylene from a process gas stream two general typesof processing systems are used, one known as a front-end process, theother as a tail-end process. In the front-end process, a full-rangestream containing both light and heavy components ranging from hydrogenup to C 's and heavier is processed over a fixed bed of selectivehydrogenation catalyst, the partially sulfided nickel catalyst beingpreferred. This catalyst is operated to effect complete removal ofsimple acetylene and removal of a majority of the methylacetylene andpropadiene. However about one-half of the butadiene is alsohydrogenated. In the tail-end process, the full-range stream is firstfractionated; then the acetylenes are removed from the individualconcentrated streams by reacting these alkynes with hydrogen overselective hydrogenation catalysts.

Partially sulfided, reduced nickel oxide catalysts normally do notperform well in tail-end processes where less hydrogen is present. Hencethey are used in front-end processes. However since these catalysts alsopromote the hydrogenation of butadiene, they cannot be used whenbutadiene is required unless the C stream is first removed. In suchinstances a depropanizer is generally employed.

It is to be understood that regardless of the process used, there is alimit to the concentration of acetylene that can be processed by normalmeans. This is due to the fact that hydrogenation of acetylene is anexothermic reaction resulting in a temperature rise across the catalystbed. With high levels of acetylene, say over 1 percent, the temperaturereaches a level where the catalyst is non-selective and loss of productoccurs. Under such conditions one of several methods is generallyemployed. The product stream can be recycled; the acetylene can behydrogenated incrementally in a series of reactors with cooling stagesbetween them; or olefin streams containing more than 1 percent acetylenecan be hydrogenated using isothermal reactors.

SUMMARY OF THE INVENTION In accordance with the practice of thisinvention a process is employed for hydrogenating acetylenes in processgas streams without first removing the C stream. In addition the processpermits the removal of acetylene by hydrogenation when more than 1percent is present in the process gas stream without isothermaloperation or the use of recycle. This invention therefore provides aprocess for the removal of butylene, butadiene, propylene, and ethylenefrom a process gas stream also containing acetylene and methylacetyleneand propadiene by hydrogenating the acetylenes with minimalhydrogenation of butylene, butadiene, propylene, and ethylene. By theprocess of this invention the process gas stream is passed through a bedof a low surface area, partially sulfided, selective hydrogenationcatalyst containing nickel, cobalt, and chromium. The partially sulfidedcatalyst is treated with an additional quantity of sulfur, that is 15 to1,500 ppm by volume of a gaseous sulfur compound, so that the acetylenesare incompletely hydrogenated, the butadiene thus remaining unsaturated.Subsequently the gas stream is separated into C C and C, fractions, andthen the C C and C fractions are conducted to separate hydrogenationzones.

DETAILED DESCRIPTION OF THE INVENTION It is normal practice when usingpartially sulfided, reduced nickel oxide catalysts to incorporate in theprocess gas feed stream a small quantity of a gaseous sulfur compoundsuch as hydrogen sulfide, carbonyl sulfide or a gaseous mercaptan. Thequantity, as sulfur, used varies from. 1 to 10 ppm. This invention isbased on the discovery that if the quantity of gaseous sulfur compoundin the process gas stream exceeds 10 ppm, say 15 to 1,500 ppm based onsulfur, the selectivity of the partially sulfided, reduced nickel oxidecatalyst is improved relative to butadiene. With this improvement it ispossible to hydrogenate acetylene without hydrogenating butadiene ifacetylene is not completely hydrogenated, for instance if 0.01 to 0.1volume percent acetylene remains in the stream. This amount of acetylenecan then subsequently be completely hydrogenated by the process of theinvention.

The partially sulfided, reduced nickel oxide catalyst employed in thefirst hydrogenation reactor, termed front-end converter, should have alow surface area. The low surface area not only enhances the desiredselectivity, but permits the addition of less sulfur, generally as a gassuch as hydrogen sulfide. A low surface area is best obtained by the useof a carrier, heat treated to reduce its surface area to below 5 squaremeters per gram. Preferred carriers to be so-treated are the usualalumina and silica-alumina catalyst bases. Such bases are well known andneed not be discussed at length herein. Desirable carriers to be heattreated are disclosed in U.S. Pat. No. 3,155,739. The catalyst is thenformulated to deposit on the carrier 0.5 to 2.0 weight percent nickel,0.1 to 0.4 weight percent cobalt, 0.02 to 0.1 weight percent chromium,and 0.5 to 1.0 weight percent sulfur, these percentages being based onthe total catalyst. A desirable catalyst is one having 1 weight percentnickel, 0.1 weight percent chromium, 0.4 weightpercent cobalt and 1weight percent sulfur deposited on a silicaalumina carrier having asurface area less than 5 square meters per gram. The preparation of thisand other catalysts employed in this process will be obvious to thoseskilled in the art.

A quantity of catalyst was prepared in accordance with the abovedescription by immersing in an aqueous impregnating solution a suitablecatalyst carrier, desirably a mixture of about 50 percent alumina andabout 50 percent silica in the form of a %-inch diameter spherepreviously heated to a temperature above about 2,000 F. to lower itssurface area below 5 square meters per gram. The impregnating solutionwas prepared by dissolving metallic nickel and cobalt in an aqueousnitric acid solution to which chromic acid anhydride had been added. Amolecular equivalent (based on metal content) of sulfuric acid was addedand the spheres immersed, drained, dried and calcined at a temperatureof about 800 F. for about 12 hours.

Catalysts containing various concentrations of catalytic metals wereprepared as above and were evaluated by loading them into an isothermalreactor and after reducing them with hydrogen at 700 to 900 F. passingvarious gaseous mixtures over the catalysts at a variety of pressures,temperatures and space velocities to measure the relative hydrogenationrates with various concentrations of sulfur in the feed gas.

EXAMPLE A A catalyst was prepared by the above procedure such that itcontained the following concentrations of catalytic metals, expressed asthe pure metal: 2.5 weight percent nickel; 0.05 Weight percent cobalt;0.01 weight percent chromium and 1.1 weight percent sulfur. Thiscatalyst was loaded into an isothermal reactor and after reduction withhydrogen a process gas of the following composition was passed over thecatalyst at selected condition: I 1.4 volume percent hydrogen, 0.2volume percent acetylene, 29.8 volume percent ethylene, 10.7 volumepercent propylene, 1.3 volume percent butadiene, 2.9 volume percentbutylene and 43.7 volume percent nitrogen. One run was made with theabove feed gas, as is, and a second run was made with the above feed gasto which 1 to 2 ppm by volume of a gaseous sulfur compound was added.Results of this test are shown in Table I.

EXAMPLE B EXAMPLE C A third test was performed using another similarfeed gas containing 0.5 volume percent acetylene and 1.0 volume percentbutadiene and 750 ppmv of a sulfur compound, hydrogen sulfide, which wasprocessed over a catalyst essentially the same as described in ExampleA, with the results also shown in Table l.

TABLE I (1 11,; Pres- Exit loss, S 111 food, sure, Tomp., S.V., C 11percent Example p.p.m.v. p.s.i.g. F. hrs." 1) p.m.v volume A 0 215 1,500 31 67. 5 l-2 00 .215 1, 500 200 22. 4 .13 370 350 450 2, 000 12 (i8.0 370 850 420 2, 700 580 0 C- 750 175 510 1,000 10. 0

The tests reported in Table 1 show that only very small concentrationsof sulfur in the feed gas have a very significant effect on the degreeof acetylene removal. They also show that with properly selectedoperating conditions and feed sulfur concentrations, catalystselectivity can be adjusted such that no measurable hydrogenation ofbutadiene will occur even at activity levels that remove better than 90volume percent of the acetylene. This discovery is the basis of theinvention.

Performance required of the catalysts for subsequently purifying theconcentrated C C and C streams, termed tail-end catalysts, is nodifferent from that currently practiced and any one of severalcommercially available catalysts could be used. However, in the detaileddiscussion of the invention we relate preferable compositions of thesecatalysts.

The process of this invention can perhaps best be understood byreference to the process illustrated in the attached schematic drawing,which is illustrative only since various pieces of equipment such aspumps, valves, and the like have been omitted, as will be apparent, andsome commercial processes may vary somewhat from that shown. However therelative positions of the catalytic converters generally are shown.

The FIGURE shows a fiow diagram of a preferred embodiment of theinvention with numerals being used to identify the various gas streams.

Referring to the FIGURE, a process gas mixture, stream 1, such as thestream resulting from the pyrolysis of a naphtha feedstock, havingpreviously been treated for the removal of an aqueous phase, a liquidhydrocarbon phase, and scrubbed free of acid gases, is conducted throughcompression stages to bring the pressure to that desired forhydrogenation of the acetylenic compounds. The pressure maintained bythe third stage compressor shown in the FIGURE is generally 200 to 500psig, although pressures of 50 to 1,000 psig can be employed. These samepressures are attained by the fourth stage of compression, such beingthe system pressure.

The pyrolysis gas feed stream contains predominantly olefins andgenerally 10 to 30 volume percent hydrogen. Specifically, in addition todesired unsaturates such as ethylene, propylene, butylene, and butadieneto be recovered, the feed gas stream contains acetylene,methylacetylene, and sometimes vinylacetylene, as well as propadiene,and frequently isoprene and ethylacetylene. Since process gas stream 1has not been previously fractionated, the stream also contains a largeexcess of hydrogen relative to the acetylenes and propadiene which areto be hydrogenated. The hydrogenation process accordingly must be one bywhich the undesirable alkynes and dienes are hydrogenated without alsohydrogenating ethylene, propylene, butylene, and butadiene. In thefront-end catalytic converter therefore a catalyst must be employedwhich promotes the hydrogenation of acetylene not only withouthydrogenating ethylene and propylene, but without hydrogenatingbutadiene. As indicated hereinbefore, when a gaseous sulfur compound isadded to the stream, the partially sulfided, reduced nickel oxidecatalyst has a selectivity such that it performs this function. Hence asshown in the drawing, hydrogen sulfide is introduced into the front-endconverter, the quantity being the afore-mentioned 15 to 1,500 ppm basedon the precess gas stream.

The third stage of compression partially accomplishes the heating of theprocess gas stream to temperature conditions employed in the front-endcatalytic converter with the remainder supplied from an external source,such as a steam preheater. The process gas stream and hydrogen sulfideare conveyed to the top of the front-end converter and passed downwardlythrough the catalyst bed at a temperature of 250 to 600 F., desirably350 to 550 F. Space velocities employed in the front-end converter arein the range of 3,000 to 5,000, depending on system pressure, anddesirably 4,000 volumes of gas per volume of catalyst per hour (VVl-l)at pressures above 150 psig.

1n the front-end converter the partially sulfided nickel catalyst makesit possible to reduce the acetylene value to a low level withouthydrogenating the butadiene. The resulting gas from the front-endconverter flows to the fourth stage of compression and after additionalprocessing such as drying and cooling is then conveyed to a debutanizer.1n the debutanizer a separation is made between C s and a butane and alighter gaseous fraction. The C fraction is removed as stream 6. Theoverhead stream containing C, through C components is further cooled andthen introduced to a demethanizer. In this fractionation column the C Cand C components are separated from an overhead stream 2 containinghydrogen, carbon monoxide, and methane. The C -C stream is furtherfractionated in a deethanizer and a depropanizer as shown in the drawingso that separate C C and C fractions can be withdrawn as streams 3, 4,and 5, respectively, and can be further hydrogenated as illustrated.

The C fraction, stream 3, is admitted to the tail-end catalyticconverter where the remaining acetylene is selectively hydrogenated sothat only an ethylene-ethane stream remains, to be separated in theethylene splitter. The C, tail-end converter is operated at atemperature of 90 to 400 F., usually 100 to 250 F., and hydrogen isadded (not shown). The quantity of hydrogen added is equal to l to 2%times the stoichiometric amount required to hydrogenate the acetylene inthe stream to ethylene. The ethyIene-hydmgen stream is passed throughthe catalyst bed at space velocities of 1,000 to 5,000 VVl-l, preferably4,000 at typical system pressures of 300 to 400 psig.

Obviously a selective hydrogenation catalyst must be used in C tail-endconverter so that little or no ethylene is hydrogenated. A desirablecatalyst can be made by the process described in U.S. Pat. No.3,471,583. Briefly the catalyst is made by impregnating active aluminawith a sufficient quantity of a heat decomposable nickel salt to form 1to 5 percent nickel-alumina spinel, and heating the impregnated aluminato a spinelization reaction temperature of 1,800 to 2,400 F. with oxygenavailable. This heat treatment decomposes the nickel salt, and withoxygen present, forms 1 to 5 percent nickel-alumina spinel throughoutthe alumina by the end of the heat treatment. The nickel spinel-modifiedalumina is then impregnated with 0.01 to 5 percent, preferably 0.01 tol, of a heat decomposable palladium salt. The resulting composition isthen calcined (800 to l,000 F.) to form a palladium hydrogenationcatalyst. Thus the aluminum oxide matrix is modified by nickel aluminaspinelization so that the nickel alumina spinel structure constitutes lto 5 percent, preferably 3 percent, by weight of the alumina spinelmatrix and its surface area is reduced to 25 to square meters per gram.More than 5 percent spinel can be used in the modification; however, noadvantages are achieved thereby. For example, a large modification of,say 5.0 percent, is no better than a slight modification of 1 percent.

As can be seen from the drawing, a purified C fraction has now beenobtained without significantly hydrogenating ethylene and butadiene.Similarly the C effluent, stream 4, from the depropanizer is selectivelyhydro-treated with l to 2% times the stoichiometric amount of hydrogenrequired to hydrogenate methylacetylene and propadiene to propylene sothat a pure propylene stream can be withdrawn from the top of thepropylene splitter. The required hydrogen is admitted with the processgas to the inlet of the C, tail-end catalytic converter and passedthrough the catalyst bed at 100 to 700 F., desirably to 350 F. Thestream is processed through the C, tail-end converter at a somewhatlower space velocity of 1,000 to 4,000, generally 2,000 VVl-i at typicalsystem pressures of 250 to 350 psig.

The catalyst employed in the C, tail-end converter is a high surfacearea palladium-on-alumina selective hydrogenation catalyst. Suchcatalysts can be made by U.S. Pat. No. 3,420,618. The alumina supportacting as a carrier for palladium in the catalysts employed herein isactivated alumina; that is, aluminum oxide which has been calcined orotherwise heated with steam or air, etc., to raise its surface area toabove 100 square meters per gram, preferably in the range of 100 to 450square meters per gram. The amount of palladium used is 0.01 to 5 weightpercent based on the total weight of catalyst, preferably 0.01 to 1.

At this point we have obtained purified C, and C fractions still withoutexcessive loss of ethylene, propylene and butadiene. Simultaneously withseparation and purification of the C and C fractions in those plantsemploying recovery of butadiene the C fraction is further fractionatedinto concentrated butylene and butadiene fractions and the butadienefraction is purified by the addition of an appropriate amount ofhydrogen and processing it over a selective hydrogenation catalyst,usually a noble metal such as platinum or palladium on a speciallyprepared support material usually all or predominately alumina. In thispurification step, impurities such as ethylacetylene and vinylacetyleneare removed with only minor losses of butadiene.

It can be seen that by the process herein ethylene, propylene, butylene,and butadiene streams can be obtained without isothermal operation,without recycle of process gases, and without first fractionating theprocess feed gas stream 1. The advantages will be even more evident froma consideration of a commercial application. Specific data resultingfrom the treatment of an effluent gas stream from high-severitypyrolysis of a liquid feedstock is shown in Table II, III, and [V whichfollow. In these table front-end purifica- TABLE II.STREAM COMPOSITIONS,FRONT-END PURIFICATION Third stage Demetharuzer Deethamzer DepropamzerDeproparuzer casphtter compression gas overhead overhead overheadbottoms teed Lb.- Per- Lb.- Per- Lb.- Per- Lb.- Per- Lb.- Per- Lb.-Perrnol./ cent mol.] cent mol.] cent mol.] cent mol./ cent mol.] centComponent hr. mol. hr. mol. hr. mol. hr. mol. hr. mol. hr. mol.

Hydrogen Carbon monoxide Total dry gas..... 9, 606. 31

Pressure, p.s.i.a 200 325 I Less than 8 p.p.m. (mold).

b Less than 80 p.p.m. (mol.).

" Less than 1 p.p.m. (mol.). 4 Less than 5 p.p.m. (mol.).

tion (Table II) and tail-end purification (Table III) are compared withthe process of the invention (Table IV).

In Table II are listed compositions of various streams numbered as inFIG. 1 showing the effects of using only a front-end catalyst to obtainless than ppm (mol) acetylene in the final ethylene product. Thisprocess requires a guard tail-end reactor on the C, fraction if a highpurity propylene product is to be obtained. The primary disadvantages ofthis process are the high ethylene and butadiene losses, and the largervolumes of catalysts that are required.

i Table lIl areTstEd aiaia'swarwtievafians streams using tail-endprocessing only to obtain both high-purity ethylene and propylene withthe impurities reduced to the same level as in Table ll. This processeliminates loss of butadiene but results in significant ethylene andpropylene losses, especially in those plants employing high-severitypyrolysis processes, due to the high level of impurity in the feed plusthe need to operate the unit within very close limits to avoid hightemperatures in the catalyst bed which produce mqssits idsu n It isobvious that this invention applies to process gas streams which notonly contain ethylene and propylene but butylene and butadiene as well.When only ethylene and propylene are present in the gas stream,conventional frontend or tail-end purification processes can be usedexcept in the case of high-severity pyrolysis. Excessive losses ofethylene and propylene could occur as a result of high levels ofimpurities causing the catalyst beds to reach non-selective conditions.Also, heretofore when butylene and butadiene have also been present ithas been necessary to remove this C stream prior to hydrogenation whenit was desirable to use front-end hydrogenation. Table II illustratesthe effect of the front-end purification system on the process gasstream when the C stream is not removed.- The acetylene was reduced from107.59 pound mols per hour to 0.04 and methylacetylene and propadienewas reduced from 57.64 and 28.82 pound mols per hour to 0.17 and 1.02pound mols per hour, respectively. However, at the same time, butadienewas hydrogenated so that its concentration dropped from 115.28 poundmols per hour to 31.01, a loss of 73 percent. As can be seen from TableTABLE IIL-STREAM COMPOSITIONS, TAIL-END PURIFICATION (1) (8) Third stageDeethanizer Depropamzer Deproparuzer compressiongas overhead overheadbottoms Czsplitterfeed casplitterieed Per- Per- Per- Lb.- Per- Per- Per-Lb.- cent Lb.- cent Lb.- cent mol.] cent Lb.- cent Lb.- cent Componentmol./hr. mol. mol./hr. h mol. mol./hr.

Hydrogen 1,327. 59 :Carbon monoxide 6. 72

Methane 660.91 6.88 Acetylene- 107.59 1.12 005.84 41.70 1,103.77 11.4957.64 0.60 28.82 0.30 891.48 19.69 204.61 2.13 115.28 1.20 22.09 0.2321.13 0.22 11.53 0.12 25.94 0.27 7.69 0.08 0.96 0.01 6.72 0.07

Total drygas-. 9,606.31 100.00 5,217.20 100.00 2,182.55 100.00 158.50100.00 5,217.20 100.00 2,182.55 100.00

Pressure, p.s.i.a 200 325 300 250 250 350 I Less than 8 p.p.m. (mol.).11 Less than 1 p.p.m. (mol.). Less than 5 p.p.m. (mol.).

Table IV shows the stream compositions that result from the use of theprocess of the invention. This process requires less total catalyst,smaller or fewer reactors, and offers greater operational flexibility,plus significant savings from reduced losses of ethylene, propylene andbutadiene through undesired non-selective hydrogenation.

ll both butylene and butane increased but the value of those productsare considerably less than that of butadiene. To avoid this loss ofvaluable product it is customary to use a frontend depropanizer and passonly the C and lighter fraction over the front-end hydrogenationcatalyst. This greatly adds to the cost of the plant because thepyrolysis product TABLE IV.STBEAM COMPOSITIONS, INVENTION PROCESS ,(1)(a) (a) (1) (a) Third stage Deethamzer Depro anizer DepropamzerCzfractionator Calractlonator compression gas overhead over ead b tomsfeed feed Lb.- Per- Lb.- Per- Lb.- Per- Lb.- Per- Lb.- Per- Lb.-Permo1./ cent mol.] cent mol.] cent mol.} cent mol.] cent mol.] centComponent hr. mol. hr. mol. hr. mol. hr. mol. hr. mol. hr. mol.

.Hydrogen 1,327. Carbon monoxide 6.

Total dry gas--. 9, 606. 31 100. 00 5, 217. 100. 00

must be refrigerated and reheated when a front-end depropanizer is used.

The tail-end purification process was developed and is used as animprovement over the front-end process in that bu tadiene is conservedand the need for reheating the entire gaseous pyrolysis product afterrefrigeration is eliminated. However, the operational flexibility ofcommercial catalysts used in tail-end purification is limited and thehigh levels of impurities resulting from high-severity pyrolysis exceedsthe limits ofgood operation with these catalysts.

The highly exothermic hydrogenation reaction raises the temperature ofthe tail-end catalysts above the limits of selective hydrogenation ofproduct olefins, i.e., ethylene and propylene. Because this processrequires that hydrogen be added to the feed streams, operation atconditions of poor selectivity results in excessive hydrogen usage whichis a valuable by-product. It also results in erratic performance in thatif too much hydrogen is consumed in the hydrogenation of ethylene andpropylene, there is an insufficiency of hydrogen for hydrogenation ofacetylene or methylacetylene and propadiene, permitting high levels ofimpurities to remain in the product.

As shown in Table IV the process of this invention eliminates orminimizes the problems associated with either the front-end or tail-endpurification processes in that it permits use of a front-end catalyst topartially remove undesirable impurities but at conditions that conservebutadiene to an extent that need for a front-end depropanizer, with itsattendant higher capital and operating costs, is eliminated. Bypartially removing impurities over the front-end catalyst, theirconcentration in the feed streams to the tail-end converters are reducedto a level that is well within established limits of good operation.

Also, by having to remove only a portion of the impurities, lesscatalyst of each type is required than if either front-end or tail-endprocessing were used alone. For example, the volumes of catalyst, incubic feet, shown in Table V would be typical for current commercialpractice with the streams given in Tables ll, Ill, and IV.

The front-end process requires a total of three reactors, two for thefront-end catalyst and one for the C fraction guard unit. The tail-endprocess requires a total of six reactors, three each for the C and Cfractions. The process of the invention requires a total of fourreactors, two for front-end catalyst and one each for the C and Cfractions.

A comparison of total yields of the more valuable products from theselective hydrogenation of the same quantities of the feed as given inTables II, III and IV, shown in Table VI, vividly illustrates theprimary economic advantages offered by the process of the invention:

TABLE Vl Product Process Million lbs/Yr Front-End Tail-End InventionEthylene 861.7 870.6 883.9

Propylene 638.8 638.8 646.4 Butylenes 28.5 9.8 9.8

Butadiene 13.3 49.3 49.3

One acquainted with the value of these commodities can quite easilydetermine that the process of the invention permits recovery ofadditional product having an aggregate value that ranges from 0.45 to3.5 million dollars per year depending on the process with which it iscompared.

In summary, it can be said that the process of the invention permits theprocessing of pyrolysis products that heretofore was not possiblewithout additional pre-processing of the hydrogenation unit feeds toavoid severe loss of valuable products; without significantly limitingthe operational flexibility of the plant; or without special processingprovisions that add to the cost of the operation. In addition, itpermits increased recovery of valuable products that are lost by beingnon-selectively hydrogenated to less valuable components.

What is claimed is:

1. In the process for the recovery of butylene, butadiene, propylene,and ethylene from a process gas stream also containing acetylene,methylacetylene and propadiene wherein the acetylenes and propadiene arehydrogenated by passing the process gas stream at a temperature of 200to 600 F. along with an excess of hydrogen beyond that required tohydrogenate acetylenes and propadiene in said stream through a bed ofnon-noble metal selective hydrogenation catalyst comprising about 0.1 to5 percent by weight nickel, 0.05 to 0.5 percent by weight cobalt, and0.01 to 0.1 percent by weight chromium and 0.1 to 1.0 percent by Weightsulfur supported on an alumina or silica-alumina carrier, theimprovement comprising adding to the process gas stream a sufficientquantity of a gaseous sulfur compound within the range of 15 to 1,500parts per million based on sulfur so that over the catalyst, and underthe hydrogenation conditions, 0.01 to 0.1 volume percent acetylenesremains in the gas stream due to incomplete hydrogenation of acetylenes,and effecting said hydrogenation under said conditions.

2. The process of claim 1 wherein the process gas stream passed throughthe initial selective hydrogenation catalyst bed contains more than 0.3mol percent acetylene.

3. The process of claim 1 wherein the process gas stream passed throughthe initial selective hydrogenation catalyst bed contains 10 to 30 molpercent hydrogen.

4. The process of claim 1 wherein the process gas stream passed throughthe initial selective hydrogenation catalyst bed is a gaseous product ofthe pyrolysis of naptha or other liquid hydrocarbons.

5. The improvement of claim 1 wherein the process gas stream contains0.50 to 1.5 volume percent acetylene, wherein following the partialhydrogenation thereof the stream is fractionated into C C and Ccomponents, and wherein the remaining acetylene is hydrogenated at anelevated temperature in contact with a selective palladium hydrogenationcatalyst.

2. The process of claim 1 wherein the process gas stream passed throughthe initial selective hydrogenation catalyst bed contains more than 0.3mol percent acetylene.
 3. The process of claim 1 wherein the process gasstream passed through the initial selective hydrogenation catalyst bedcontains 10 to 30 mol percent hydrogen.
 4. The process of claim 1wherein the process gas stream passed through the initial selectivehydrogenation catalyst bed is a gaseous product of the pyrolysis ofnaptha or other liquid hydrocarbons.
 5. The improvement of claim 1wherein the process gas stream contains 0.50 to 1.5 volume percentacetylene, wherein following the partial hydrogenation thereof thestream is fractionated into C2, C3 and C4 components, and wherein theremaining acetylene is hydrogenated at an elevated temperature incontact with a selective palladium hydrogenation catalyst.